1. Field of the Invention
The present invention relates to offshore drilling and well activities preformed from a floating drilling or workover rig or vessel.
2. Description of the Related Art
Today, when an offshore sub-sea well is intervened (work performed inside the production tubing below a sub-sea x-mas tree) from a floating vessel, a high pressure workover riser system is used. Such work-over riser systems has been designed with a subsea shut off valve (LRP) and/or a BOP configuration, close to the seabed and includes a riser disconnect package (RDP), to allow for a riser disconnect close to seabed when situations call for it. On the surface, the high pressure riser is terminated in a surface test tree (series of valves) above the rigfloor. To allow for riser tension, the drilling rig's main blocks for lowering and hoisting drillpipe is used to pull tension on the workover riser. Above the surface test tree, the pressure control equipment (surface BOP) for the well operations is installed, for lubricating into the well all of the work-over tools used in the operation.
Presently such systems are either designed for use in open water (stand alone), or the workover riser is run inside a 21″ OD low pressure marine drilling riser system, which includes a 18 3/″ inside diameter subsea drilling BOP installed on top of the sub-sea x-mas tree at seabed.
If the work-over system is being used inside a 21″ drilling riser, the lower shutoff valves in the workover riser system close to seabed, are controlled independent of the drilling BOP on the outside and carry independent equipment for service of the well. To run all of this equipment is very time consuming, in that the rig crew first has to run the 21″ marine drilling riser and the 18¾″ drilling BOP and suspend this system in the drilling rig's riser tension system underneath the rig floor. Then the rig crew has to run the workover riser system inside the marine drilling riser all the way to seabed and suspend this riser to the outer drilling subsea BOP in the lower end and suspend this riser system in the rig's main drilling hook by help of an elevator or lifting frame in the upper end. In doing so, the main travelling blocks/hook is occupied and will prevent the rig from being able to run jointed pipe into the workover riser.
If the high pressure riser is run as a stand alone system in open waters, the subsea blowout preventer (BOP) and the riser disconnect package (RDP) is installed on top of the subsea x-mas tree. This riser system is to date not intended for use with jointed drillpipe but intended for extending the production tubing up to the drilling rig's work deck or rigfloor, so that wireline and coil tubing can be run into the well. This riser system is then hung off in the rig's drilling riser tensioning system and/or in the drilling hook with the help of an elevator or lifting frame. The surface BOP's for the workover riser system is then installed above the rig floor and above the elevator to the rigs main hoisting system. This will also prevent the rig from being able to run jointed drillpipe into the well, since the equipment for running jointed pipe is occupied holding tension in the riser system. Hence with prior art, it is not possible to change from running wireline or coiled tubing equipment into the well, into the process of running jointed drillpipe into the well or vice versa, without having to change out the whole riser system or disconnecting the riser from the production sub-sea x-mas tree.
U.S. Pat. No. 5,676,209 describes a low pressure drilling riser system which comprises 2 BOP stacks. The upper BOP stack is submerged below the wave affected area closer to sea level and the lower BOP placed at seabed. A marine drilling riser extending back to the drilling installation can be disconnected from the upper BOP stack so that the riser will be free standing in the ocean due to air-cans or flotation elements installed below and adjacent to the upper BOP. This riser system is fundamentally different from the high pressure drilling and workover riser specified in the present invention in that the riser specified in U.S. Pat. No. 5,676,209 is a low pressure riser with high pressure kill and choke lines running outside and parallel to the main drilling riser bore. This riser would not be able to tolerate high pressures from the well. In order to perform workover operations, a complete new workover riser would have to be installed inside this riser and extending down to the subsea BOP. Hence this prior art would not introduce any added benefits to the invention here described.
PCT publication WO98/58152 describes an apparatus and method for drilling sub sea wells. This apparatus has no BOP at seabed and describe an apparatus where the BOP stack is moved to higher location closer to the sea level. The system introduces large buoyancy elements or air cans which is required if the drilling riser connecting the BOP to the drilling rig has to be disconnected. A system of this nature as described in the publication, could not be connected to a sub-sea production x-mas tree and hence the production tubing for underbalanced workovers. Hence, this prior art is fundamentally different from this invention.
The Norwegian patents NO 306174 (H. Møksvold) and NO 305138 (S. Gleditsch) describes a high pressure drilling and workover riser which resembles the riser described in this invention. NO 305138 and NO 316174 describe a system where the workover coil tubing BOPs are integrated with the upper BOP which is terminated in the drilling and workover riser in the upper end. However in order to effectively be able to change between drilling with jointed drillpipe and underbalanced work with standard workover BOP equipment on a conventional drilling rig, with minimum of time used and without modifications, it will be necessary to introduce the high pressure riser extension sleeve described in this invention. The high pressure riser extension would allow for the coil tubing or wireline BOPs to be introduced only when needed and the integration process can be performed without having to disconnect the main workover riser from the sub-sea wellhead or sub-sea x-mas tree. These inventions wills hence save expensive rig time and/or prevent expensive equipment from being rigged up on top of the riser in the upper BOP when not needed.
The PCT WO 03/067023 A1 describe an arrangement and method for well completion and intervention operations where a workover riser projecting from a wellhead and up to a vessel is used, and where the upper portion of the workover riser is designed to be displaced from the upper position to a lowered position favourable for rigging work, where at least the upper displaceable portion of the workover riser essentially follows the heave motion of the vessel. This is a telescopic high pressure joint integrated with the main workover riser which is high pressure only when fully stroked out and put in tension by the rigs main hoisting equipment. This prior art is run inside a conventional drilling riser and extends all the way down to the seabed. In order to convert to drilling the whole workover riser including the sub-sea safety valves and control system for same, must be disconnected from seabed and pulled out of the well. In deeper waters this will take considerable amount of time. However integrating this prior art in the high pressure extension sleeve in this invention would allow for more easy rig-up of workover BOPs and tools on the drill floor. Time savings are hence only achieved by combining prior art with this new invention in this particular way and combination.